The present invention relates to a catalyst precursor for an activated Raney metal fixed-bed catalyst and a shaped and activated Raney metal fixed-bed catalyst. In another aspect, the present invention relates to a process for preparing a shaped and activated Raney metal fixed-bed catalyst by preparing a mixture of powders of at least one catalyst alloy and at least one binder with the addition of moistening agents and auxiliary agents such as shaping aids, lubricants, plasticizers and/or pore-producers, homogenizing and shaping this mixture to give the desired molded item, calcining the molded item and activating the catalyst precursor obtained in this way by partially or completely leaching the leachable alloying component and subsequently washing the final catalyst. The catalyst alloys each contain at least one catalytically active catalyst metal and one alloying component leachable in alkali. Furthermore, the present invention relates to the use of such a shaped and activated Raney metal fixed-bed catalyst and a method of hydrogenating organic compounds using such catalysts.
Activated metal catalysts are known as Raney catalysts in the chemical engineering field. They are used mainly in the powdered form in a large number of reactions for hydrogenating organic compounds. These powdered catalysts are prepared from an alloy of a catalytically active metal (also called a catalyst metal or Raney process metal) and another alloying component which is leachable in alkalis. The Raney process metals used are mainly nickel, cobalt, copper or iron, but also other metals such as palladium and silver may be used. The alloying component which is mainly used is aluminum, but other components can be used, in particular zinc and silicon are also suitable.
This so-called Raney alloy is first finely milled according to Raney's method. Then the aluminum is completely or partially removed by leaching with alkalis such as, for example, caustic soda solution. This process activates the alloyed powder. Due to leaching of the aluminum, it has a high specific surface area between 20 and 100 m.sup.2 /g and is rich in absorbed hydrogen. The activated catalyst powder is pyrophoric and is stored under water or organic solvents or is embedded in high-boiling organic compounds.
Powder catalysts have the disadvantage that they can only be used in batch processes and have to be isolated after the catalytic reaction by time-consuming filtration of the reaction media. Various processes for preparing molded items have therefore been disclosed which lead to activated metal fixed-bed catalysts after extraction of the aluminum. Thus, for example, coarse particulate, i.e. only coarsely milled, Raney alloys are available which can be activated by treatment with caustic soda solution. In this case, leaching and activation of the particulate take place only in a shallow surface layer whose thickness can be set by means of the extraction conditions.
The major disadvantage of catalysts prepared using this method is the poor mechanical stability of the relatively thin activated outer layer. Since only this outer layer of the catalyst is also catalytically active, this results in rapid deactivation which at best can be partially reversed by renewed activation of deeper-lying alloyed layers using caustic soda solution.
U.S. Pat. No. 4,826,799 describes the preparation of activated Raney metal fixed-bed catalysts by mixing a powder of the alloy of Raney process metal and aluminum with an organic polymer and optionally a shaping aid, shaping this mixture by extrusion or compression to give the desired molded items, and calcining the molded items in air at temperatures above 850.degree. C. This leads to a pore structure in the molded item, due to combustion of the organic additives, and to the formation of .alpha.-aluminum oxide which acts as a ceramic binder between the alloyed particles and gives the desired mechanical stability to the molded items. Then follows activation of the molded item by leaching of the remaining aluminum which was not oxidized during calcination.
The critical feature of this known process is the formation of .alpha.-aluminum oxide between the alloyed particles as a ceramic binder. .alpha.-Aluminum oxide, in contrast to .gamma.-aluminum oxide and aluminum itself, is not leachable in alkalis and is therefore not dissolved out when activating the molded item with caustic soda solution. An advance, as compared with fixed-bed catalysts in the form of coarse particled alloys, is the formation of a pore system by burning out the organic auxiliary agent. The pore system in the final catalyst enables educt molecules to diffuse into the catalyst and product molecules to diffuse out of the catalyst.
However, catalysts prepared in accordance with U.S. Pat. No. 4,826,799 also have serious disadvantages. In order to form .alpha.-aluminum oxide, the molded item has to be calcined at temperatures higher than 850.degree. C. In fact, below 850.degree. C. no .alpha.-aluminum oxide is formed at all, only .gamma.-aluminum oxide which is soluble in alkalis. The high calcination temperature leads to high energy consumption. The .alpha.-aluminum oxide used as binder is catalytically inactive and thus reduces catalyst activity. During calcination, a more or less sealed layer of this inactive material which is insoluble in alkalis is formed on the surface of the alloyed particles. This means activation of the alloy is difficult. In the final catalyst, this layer represents a barrier to diffusion of the educt molecules, which results in a further loss of activity.
The ability to be easily recycled is required of modern catalyst systems in order to protect the environment. However, processing ceramically bonded metal fixed-bed catalysts is difficult due to the insoluble ceramic binder.
I. Nakabajasi (Katal. Reakts. Zhidk, Faze 280-3 from: Ref. Zh., Khim. 1973, Abstr. No. 6B1100 (Russ) 1972) describes bonding powdered catalyst alloys with powdered alkali- and acid-resistant glass frits which melt at high temperature. After the calcination procedure, aluminum is dissolved out of the catalyst precursor as usual by leaching with alkalis. In this case, the sintered glass frit particles between the alloy particles are responsible for the mechanical stability of the catalyst. When adding a small proportion of glass powder there is the risk, during leaching, that the molded item might be dissolved. However, when adding high proportions of glass powder, alloy particles embedded in the sintered glass matrix can no longer be activated. Here then is a bulky barrier to diffusion, similar to the one described above. In addition, the temperatures required to sinter the glass frit component are always higher than 850.degree. C.
The use of low-melting glass frits (that is with a melting point below 850.degree. C.) is not possible because this type of glass frit is generally not chemically stable towards alkalis and thus is dissolved out in the activation step.
Japanese Patent JP 500 99987 describes a process for preparing metal fixed-bed catalysts based on nickel, cobalt or copper catalysts. In this case, the actual nickel, cobalt or copper catalyst is first mixed with up to 30 wt. % of a powdered metal/aluminum alloy. This mixture is then shaped to give appropriate molded items and is treated with steam at elevated temperature. This produces .gamma.-Al.sub.2 O.sub.3 which acts as a binder in these catalysts. However, analogous application of this process to the preparation of activated Raney metal fixed-bed catalysts is not possible, as already mentioned, because at the latest during the activation procedure with caustic soda solution, without which a Raney catalyst cannot be prepared, the .gamma.-Al.sub.2 O.sub.3 binder would be dissolved out. The process presented in Patent JP 500 99987 is thus not suitable for preparing shaped and activated Raney metal catalysts.